A laser beam can interact with a surface in a number of ways to change the surface properties, including light absorption, photon scattering and impact. For example, a surface may be burned by an intense laser beam. Some surface particles may be ablated from a surface by the impact of a laser beam. Therefore, a surface can be treated with one or more proper lasers to achieve certain effects that may not be easily done with other methods.
One example is described in a copending U.S. patent application Ser. No. 08/729,493, pending, titled "LASER METHOD OF SCRIBING GRAPHICS", which is a continuation-in-part of the U.S. patent application Ser. No. 08/550,339, filed on Oct. 30, 1995 by the present inventors. The disclosure of the application "LASER METHOD OF SCRIBING GRAPHICS" is incorporated herein by reference. This application describes the use of lasers to form graphics on various materials by controlling the energy density per unit time. The graphics can be patterns, images, letters, and or any other visual marks.
Although other traditional methods, such as dyeing, printing, weaving, embossing and stamping, have been widely used, laser methods appear to have certain advantages in producing complex and intricate graphics on the materials. This is at least in part because many of the traditional methods lack the necessary registration and precision to insure that minute details of the graphics are accurately and repeatably presented on the materials. In addition, laser methods obviate many problems associated with the traditional methods such as high cost of equipment manufacturing, equipment maintenance, and operation, and environmental problems. A detailed description of laser methods for scribing graphics is disclosed by the present inventors in the above-referenced U.S. Patent Application "LASER METHOD OF SCRIBING GRAPHICS".
The extent of laser interaction with a material can be characterized by a number of parameters, including spot size, intensity, power, etc. The inventors found that one preferred parameter is the energy density per unit time ("EDPUT") defined as ##EQU1## wherein the projection speed is the speed at which the scanning beam spot moves on a treated surface. The laser operational parameters, i.e., laser power, beam spot size, and the scanning speed, should be adjusted to achieve an optimal EDPUT for a specific material and a particular scribing requirement. If the EDPUT is too high, the surface may be carbonized, burned or melted; if the EDPUT is too low, the effect of laser treatment may not be sufficiently visible.
The inventors recognized that lasers can also be used to treat a material surface in order to achieve a certain texture or appearance of the surface. Many materials used by the fabric industry are treated for this purpose.
Denim fabrics may undergo a sandblasting process to obtain a worn look. Denim jeans are often sold with a worn look in the upper knee portions and back seat portion. The effect is similar to a feathered or shadowed look in which the degree of the worn look continuously changes along the length and width of the seemingly "worn" areas.
A sandblast treatment conventionally abrades the jeans with sand particles, metal particles or other materials at selected areas to impart a worn look with a desired degree of wear. This process blasts sand particles from a sandblasting device to a pair of jeans. The random spatial distribution of the sand creates a unique appearance in a treated area. Denim jeans and other clothing treated with such a sandblast process have been very popular in the consumer market.
However, the sandblast process has a number of problems and limitations. For example, the process of blasting sand or other abrasive particles presents significant environmental issues. A worker usually needs to wear protective gear and masks to reduce the impact of inhaling any airborne sand or other abrasive particles that are used. The actual blasting process typically occurs in a room which is shielded from other areas in a manufacturing facility. Further environmental issues arise with the clean-up and disposal of the sand. In practice, undesired sand is rarely completely eliminated from the pockets of the denim jeans or jackets.
The sandblasting process is an abrasive process, which causes wear to the sandblasting equipment. Typically, the actual equipment needs to be replaced as often as after one year of normal operation. This can result in added capital expense and installation.
In addition, the actual cost of the sandblasting process is estimated as high as several dollars per unit garment depending upon capacity utilization. This high cost is at least in part due to the labor involved, the cost of the equipment repair or continual purchase, the environmental clean-up required, the sand used, and actual yield of the goods.
Furthermore, the sandblasting process can adversely affect the strength and durability of the finished goods due to the abrasion of the sand or other particles that are used.
Despite the above problems and limitations, the sandblast process is still in wide use simply because there is no other alternative technique that can economically produce the desired surface appearance of the sandblast treatment. In view of the above, the inventors found it desirable to replace the sandblast process with a new environmentally friendly process which is capable of producing the "sandblast look", while reducing the cost and maintaining the durability of the finished goods.
In recognition of the above, the inventors invented laser scribing methods to achieve the worn look on fabrics such as denim, the details of which are disclosed in the above incorporated U.S. Patent Application "LASER METHOD OF SCRIBING GRAPHICS". For example, one method is to drape the denim over a cone, cup or wedge surface that is positioned relative to a laser with a beam scanning device so that the focused beam projects different spot sizes at the different location of the surfaces due to a distance variation from the focusing distance. Thus, the beam intensity changes with the beam location on the surface. Accordingly, the degree of laser scribing or the EDPUT on a piece of fabric on the surface changes. The laser sweeps over the surface to scribe a predetermined pattern such as a solid pattern or a pattern with closely spaced lines. The locations on the surface that are closest to the focusing distance to the laser receive the highest beam intensity or EDPUT and hence have the more worn appearance. Conversely, the locations on the surface that are most out of the focusing distance experience the lowest beam intensity or EDPUT and hence have the least worn appearance. This technique has the effect of continuously changing the laser focus as the laser beam scribes a pattern on the material. Alternatively, the laser focus can be changed with respect to a flat work surface to achieve the same effect.
Another method previously disclosed by the inventors relies upon using a reference EDPUT grid over a treated area. Again, a pattern is scribed in the treated area on a material surface. However, the operating parameters of the scanning laser are changed along the grid with a predetermined EDPUT distribution to achieve a desired effect, such as a feathered look.
A third method uses a pattern having a series of lines with continuously increasing or decreasing line spacing and thickness to achieve the feathered or worn look. Alternatively, a radial gradient pattern can also be created by the scanning laser with predetermined EDPUTs to produce a desired fabric appearance.
The results of the above laser scribing techniques produced a "feathered" look that approximates the worn look achieved from the sandblast process. However, the treated fabric does not exactly replicate the well-recognized worn appearance produced by the sandblast process. This is at least in part due to the fact that the laser scribing techniques are essentially based on scribing with a regular pattern rather than the spatially random hits of the blasted sand or other abrasive particles. Also, the above laser methods usually have long processing cycle times. For example, a typical cycle time of 6 minutes or more is needed to process an oval section of 21 inches in length.
Another limitation is that laser scribing requires a certain pattern with certain laser operating parameters (e.g., EDPUT) for a particular fabric in order to create a worn look. Since this can change from one fabric to another, the methods are very material specific and are not universally applicable to different fabrics.